1. Field of the Invention
The present invention relates to a phosphorus luminescent display tube which gives no noise trouble and the like to the electronic equipment positioned in the vicinity thereof.
2. Description of the Prior Art
Generally, a phosphorus luminescent display tube can be driven on a low voltage, being low in power consumption. In addition, such emits a very bright blue-green light which is easy to see. Thus, such is extensively used for various display devices.
Basically, the phosphorus luminescent display tube is composed of a filamentary cathode (hereinafter referred to as a filament) for emitting thermions when energized and heated in a vacuum inside the tube, and a pattern display section having a plurality of segment anodes (for instance, seven segment anodes arranged in the form of the numeral 8) each coated with a phosphor layer and selectively kept positive with respect to the abovementioned filament, whereby the thermions emitted from the filament impinge on the anodes kept positive with respect to the filament to make them luminesce for display. In the case of the multi-digit display tube, a plurality of pattern display sections are provided which are adapted to selectively display a plurality of patterns, figures and the like for multi-digit display.
Also, most of the phosphorus luminescent display tubes of the above type are usually of the so-called triode type in which the thermions emitted from the filament are accelerated and controlled so that they may impinge uniformly on each pattern display section and at the same time desired pattern display sections may be selected for display by providing, for instance, a mesh-shaped control electrode between the filament and each pattern display section or all the pattern display secitons in common.
Moreover multi-digit phosphorus luminescent display tubes are generally classified into two types in terms of the drive system, namely, the dynamic drive type and the static drive type. In the dynamic drive type tube, the corresponding segment anodes of the respective digits or pattern display sections are connected in common, and a plurality of control electrodes, one for each digit or pattern display section, are provided electrically independently of one another. Thus, display signals are given to the corresponding segment anodes of the respective digits while digit-selecting signals are given to the control electrodes in a time-sharing manner so that a desired display may be obtained. In the static drive type tube, a fixed control voltage is given to the control electrodes connected electrically in common while display signals are given to the segment anodes of the respective digits on a digit-by-digit basis so that a desired display may be produced. Either of these systems may be chosen according to the number of digits and the ambient operating conditions.
In the dynamic drive system, for instance, the digit-selecting signals are fed as pulses. These pulse signals generate, during the rise time or fall time, high frequencies which may cause noises in the external equipment. Therefore, the phosphorus luminescent display tubes for use as the display devices of radios, televisions, various measuring instruments and the like which must be prevented from noises often adopt the static drive system.
As mentioned above, such luminescent display tubes are becoming extensively used in displaying applications such as the numerical display of the results of registering, computation and the like, time display of digital time-pieces, and the display of transmitting or receiving frequencies and channels of the radio equipment including radios and televisions.
With the spread of the application range of such luminescent display tubes, it has been found that such have the following disadvantage.
When the phosphorus luminescent display tube of the triode type mentioned above is used for channel display or time display in the vicinity of an electronic equipment, for instance, a television, whose working frequency ranges from 100 MHz to several hundred MHz, the display tube emits noises falling within the above-mentioned frequency band, and thereby generates noise troubles to the electronic equipment, for instance so as to cause flicker in the television picture. The occurrence of the above-mentioned noises is very frequent especially when there are a number of segment anodes to which the anode potential is not given.
Accordingly, the inventors of the present invention have carefully investigated the causes of the occurrence of the above-mentioned noises and, as a result, reached the following conclusion.
No such conclusion will be hereinafter described with reference to FIG. 1 which shows the basic arrangement of the phosphorus luminescent display tube of the triode type mentioned above. In FIG. 1, reference numeral 1 designates an insulating substrate made of, for instance, glass or ceramics, on which are provided in a laminated manner wiring conductors 2, insulating layers 3 each having a through-hole 4, and segment anode conductors 5 electrically connected to the corresponding wiring conductors 2 through the corresponding through-holes 4. The segment anode conductors 5 are coated on the upper surfaces thereof with phosphor layers 6, respectively, to form segment anodes 7a and 7b.
In addition, a filament 8 is provided opposite to and above the segment anodes 7a and 7b for emitting thermions .crclbar. when energized and heated. Between the filament 8 and the segment anodes 7a and 7b, there is provided a control electrode 9 kept at a positive potential substantially equal to the anode potential. The control electrode 9 may be formed, for instance, in a mesh shape so that the emission of light from the segment anodes 7a and 7b may be clearly seen. Though not shown, a front bulb with a transparent observation window is bonded to the insulating substrate 1 along the peripheries thereof to form a high-vacuum casing for airtightly sealing the above-mentioned electrodes so that these electrodes may be externally energized.
With the above-mentioned construction, this phosphorus luminescent display tube may be operated as follows.
When the segment anode 7a is given an anode potential through the wiring conductor 2 and, on the other hand, the segment anode 7b is not given the anode potential but is instead kept at about the same potential as the filament 8, thermions .crclbar. emitted from the filament 8 are accelerated by the control electrode 9 which is kept positive with respect to the filament 8, impinging on the phosphor layer 6 of the segment anode 7a to thereby make the segment anode 7a emit light. Meanwhile, thermions .crclbar. moving toward the segment anode 7b to which the anode potential is not given are first accelerated by the control electrode 9 kept positive with respect to the filament 8, moving further toward the segment anode 7b through the mesh of the control electrode 9. However, since the segment anode 7b is kept at about the same potential as the filament 8 as mentioned above, these thermions .crclbar. cannot reach the segment anode 7b, being repelled thereby and accelerated in the reverse direction toward the control electrode 9. After thus passing through the mesh of the control electrode 9, these thermions .crclbar. are again made to reverse the moving direction thereof by the action of the control electrode 9, being accelerated toward the control electrode 9 again. These operations of thermions .crclbar. are repeated to produce electron oscillations about the control electrode 9 in the vicinity thereof, and these electron oscillations create electromagnetic waves, which are radiated outside to generate noise troubles to the adjacent electronic equipment.
More particularly, the electromagnetic waves produced by the above-mentioned electron oscillations correspond to those produced by the Barkhausen-Kurz oscillation in the case of common electron tubes. Accordingly, these waves have frequencies of the order of MHz and therefore cause noise troubles in the adjacent electronic equipment such as televisions which deal with signal components in the frequency range of about 100 to several hundred MHz.